Quality control of a product or process has become a large part of the economics of industry. A major concern of quality control is accuracy in measuring and the ability to detect the slightest fault in a variety of products and processes. Various devices are used to measure differences in weight, temperature and other dimensions. Such devices are usually nonportable, time consuming, inaccurate, invariable for use in detecting more than one object, and often incapable of giving a quantitative analysis.
Radiation detectors can be used to detect abnormalities by measuring temperature change and heat loss or gain. Radiation detectors have been used as a non-contact alternative to many temperature sensors. Infrared scanning devices have also been used to detect temperature differences between a subject and a reference as well as to measure heat loss from machinery, plumbing, electrical lines and the like. Typically such radiation detectors and infrared scanning devices employ radiation sensors which respond to changes in radiation in the order of 1/10 second. Such sensors are not only fast, but accurate and economic as operations of interest do not need to be shut down during detection.
Radiation detectors are based on the principle that the thermal radiation emitted from a subject is proportional to the temperature of the subject raised to the fourth power. The radiation emitted is also a function of the emissivity of the subject and of background radiation, but can be calibrated out for applications in which the target has consistent properties.
One type of radiation sensor is a thermopile. Thermopiles in general have been used to provide an indication of target temperature. A thermopile operates on the principle that sensed radiation causes a voltage to be produced at the thermopile output which is indicative of the difference between the hot and cold junctions of the thermopile.
One typical problem with radiation sensors such as thermopiles is their tendency to become overheated by energy trapped within the device. Such overheating and retaining of energy by the radiation sensor causes inaccuracies in the temperature readings. Many sensing applications require close range detection. A user in such a situation often runs the risk of heating or cooling the device with changing environmental conditions, which may change the cold junction temperature of the device or perhaps even distort the sensor output by causing uncontrolled thermal gradients. In addition to heat management problems, radiation sensor devices face dirty as well as harsh environments. Elaborate cooling, purging and cleaning systems have been used, but are expensive, clumsy and require maintaining close calibration.